An atheist is a person who does not believe in any gods. That is the only similarity among ALL atheists. Anything else, such as that they rely on science and logic, are beliefs held by MOST atheists. A lack of a belief in something is not a religion. Not believing in fairies is not a religion, not believing in unicorns is not a religion, and likewise not believing in gods is not a religion.
Atheism declares that there is no god. Christianity teaches that there is a God. Both cannot be true. So which one is correct, the atheist or the Christian? How you respond depends upon faith. It depends on the source of your faith. Will you have faith in self? Or in God? The Christian's source of confidence--his faith-'is an unshakable faith in God, revealed in His Word, manifested in Jesus Christ, and confirmed by His Spirit. This is not only a more defensible belief system than the atheist's; it is a far better way to live. In fact, it is the only way to really live. "Whoever has the Son has life; whoever does not have the Son of God does not have life" (1 John 5:6). http://www.sharefaith.com/guide/christian-apologetics/christian-arguments-against-atheism.html
I find it funny that most of the athesitic answers on here not only give a biased definition, but they completely deny them ever being biased even when they use the word "brainwashed". An atheist is someone who rejects the idea of a God or God's. There you go straight from a frankly offended chrsitian after reading some of these answers....not that anyone cares for the feelings of christians anymore, just for the rights of everyone/everything else...
An atheist is a person who either: 1.) believes there is no god or 2.) lacks the belief in any god. It really all depends on the atheist, as there are some atheists who will say that they believe there is no god.
And no, it is not a religion. A religion has to do with the beginning of the universe and higher powers. Even if you believe there is no god, you're simply rejecting ONE claim of how the universe started.